1. Field of the Invention
The present invention relates to a ship's rudder.
2. Description of the Related Art
As means for enhancing a propulsive efficiency of a ship, Japanese Patent Publication Laid Open No. 15895/72 discloses the provision of a fin extending horizontally from each side of a rudder body, the fins symmetrically mounted relative to the center plane of the rudder, whereby rotational energy in a propeller slip stream is recovered and converted to a forward thrust. A fin of an improved form is disclosed in Japanese Utility Model Publication Laid Open No. 28000/90. In this example, as shown in FIGS. 5(A) to (C) herewith attached, a leading edge of a rudder 1 coincides with a leading edge of a fin 2 on the center line of the rudder 1, and the leading edge of the fin has a sweepback angle of .theta.. The chord length of the fin 2 is set so that the length lr at the root of the fin is one and a half times the length lt at the tip of the fin, or more, and a plane form of taper fin, wherein the chord length of the fin decreases from the above-described root of the fin toward the tip of the fin is adopted.
In the form of fin of the above-described prior art rudder, the leading edge of the fin has a sweepback angle of 20.degree. to 50.degree., and the plane form of the taper fin, wherein a fin span decreases as it comes near to the tip of the fin, is adopted. In consequences, when an inflow attack angle of a current flowing into the fin is regarded as large to some extent, a strong fin tip vortex tends to occur at the vicinity of the fin tip compared with a case where a fin has a plane form without any sweepback angle. (see FIG. 6).
The inflow attack angle (.alpha.t) at the fin tip is decreased (.alpha.t&lt;.alpha.r) by an induced velocity of the strong fin tip vortex, whereby lift caused to the fin is decreased and drag (referred to as an induced drag) is increased. Accordingly, thrust, which is caused by the fin and which is a forward component of the lift, is decreased, with an adverse affect on propulsive efficiency.